1. Field of the Invention
This invention relates generally to the field of usage monitors for small-arms and more specifically to a device for determining wear in small-arms through data collection and statistical analysis.
2. Description of Related Art
Many devices have been proposed to monitor the number of rounds fired an automatic or semi-automatic weapon. In general these devices are meant to warn the shooter before the magazine becomes empty. Some of these devices count the number of rounds in a magazine; others assume that a full magazine has been inserted and count the number of rounds fired using a shot detector. A few devices have been proposed that record the time and date when a weapon was fired, particularly for use in criminal investigations. Yet other devices are currently in use on paint-ball guns for scoring, timekeeping and billing purposes. Although all of these devices are able to impart useful information about small-arms use over short periods none can provide information that can be related to wear of the barrel or internal mechanisms that are an essential part of any maintenance program.
Maintenance of small-arms is of particular concern to law enforcement, the military and to competitive shooters. Wear gradually degrades the accuracy of a firearm and in extreme cases can lead to the bursting of a barrel and injury to the shooter. Wear can also lead to jamming, particularly in automatic and semi-automatic firearms. Maintenance schedules based on time in service completely ignore the firing schedule of a firearm. When used in training thousands of rounds can be fired in a period of several months while in other periods a firearm may remain completely unused. A monitor that can be used to relate the firing history to barrel wear would allow maintenance to be based on usage, thereby benefiting all users of small-arms.
Some attempts have been made to record such data. In patents by Davis et al, (1975, U.S. Pat. No. 3,914,996) and by Gartz (1999, U.S. Pat. No. 5,918,304) an electronic apparatus is disclosed for determining the wear of the gun tube of an artillery weapon. Wear in an artillery gun tube is governed not only by the number of rounds fired but also by the charge, which may be varied with each round. Davis et al used a strain transducer to detect that a shot had been fired and applied a weighting function, proportional to the strain level, to determine the charge. The weighted number of shots fired was then stored in memory so that barrel wear could be estimated.
The approach of Davis et al fails to take into account the effects of temperature on barrel wear. If a series of rounds are fired the gun tube is heated and wear, which results from the abrasive properties of the propellant, corrosion by the expanding gases and thermal gradients through the tube wall, is greatly accelerated. It is also of limited applicability to small-arms where the shock and vibration of ordinary handling could produce many false counts.
In U.S. Pat. No. 4,001,961 (Johnson et al, 1977) a shot counter is attached to a firearm for use in a maintenance program. As an example, they cite the replacement of the extractor after 15,000 rounds have been fired. Firing is detected by a micro-switch on the trigger, an inductance or piezoelectric transducer in the buffer, or an inertial switch that responds to recoil. The switches complete an electric circuit containing a battery that allows an electrochemical plating process to proceed while the transducers are used in a passive system, providing the electric potential that drives the plating. Usage is monitored by comparing the thickness of the plated layer at one end of a transparent tube to a color-coded scale on or adjacent to the tube. As in the previous citation there has been no thought given to avoiding false counts through handling.
Avoiding false counts is addressed in a patent by Hudson et al (1979, U.S. Pat. No. 4,146,987). An inertial switch comprising a pivoting, eccentric mass, a mechanical counter and a spring that allows a threshold acceleration to be set. This purely mechanical system is relatively large and difficult to implement on small-arms. It is also likely to undergo a change in threshold as the contact surface between the spring and the shaft wear during use. Clearly an electronic device is preferable for use with small-arms where size and weight are important concerns.
An example of an electronic shot counter for small-arms is that patented by Home and Wolf (1991, U.S. Pat. No. 5,005,307). Two micro-switches are used to provide input to a micro-controller that counts the rounds remaining in a magazine. An LCD display is used to indicate this count. Insertion of a new magazine is sensed by the first switch and the count is reset. Firing is detected by a second switch on the gun's slide. Doubtless this device could be modified to count the cumulative number of shots fired, however, slide movement while unloaded or when chambering the first round from a new magazine will result in false counts.
A number of other patents add desirable features to the teaching of Home and Wolf. The aforementioned device cannot differentiate whether a round is in the chamber when a new magazine is inserted; Herold et al (1997, U.S. Pat. No. 5,642,581) resolve this ambiguity by allowing the user to increment the count indicated by the counting device; Villani (2000, U.S. Pat. No. 6,094,850) teaches the use of an additional switch within the chamber to automatically adjust the count. Neither device can differentiate between a round that has been fired and one that has been ejected without firing as required when a weapon is to be made safe.
Other inventors have sought to eliminate micro-switches in order to reduce cost and complexity while improving accuracy, reliability and sensor life. U.S. Pat. No. 5,406,730 (1995, Sayre) describes the use of an inertial switch in combination with an acoustic sensor to detect firing. Handling shocks cannot cause false counts because an acoustic signal must occur simultaneously before the count is incremented. Similarly, an acoustic signal from a weapon fired nearby cannot increment the count unless a simultaneous recoil is detected. Brinkley, in U.S. Pat. No. 5,566,486 (1996), discloses an inertial switch that is adjustable; this makes it possible to set the acceleration level that will trigger a count so that recoil can be differentiated from handling shock. An additional benefit of this device is it ability to be adjusted to work on weapons with different recoil characteristics. A stated use of Brinkley's shot counter is to record the number of shots fired during a firearm's lifetime for use in its maintenance.
The patent of Harthcock (1994, U.S. Pat. No. 5,303,495) teaches the use of a Hall-effect device for counting shots fired from small-arms. A micro-processor records in non-volatile memory the time and date of each shot fired along with the direction, from a Hall-effect compass, for crime lab analysis. In common with many of the previously described devices this counter cannot distinguish between the firing of a round, the chambering of the first round after the last shot in a magazine has been fired or the ejection of an unfired round.
The most technologically advanced devices for monitoring the firing of a projectile have been developed for use in paintball guns. When used in commercial applications it is important to record the number of rounds fired and the amount of time that a gun has been used. It is also desirable to provide information such as firing rate, maximum firing rate and battery condition to the user and to communicate these data, along with the gun's identification number, back to a control center. These features are all taught in U.S. Pat. Nos. 6,590,386 (2003, Williams) and 6,615,814 (2003, Rice and Marks). Both patents teach the use of a temperature sensor that is used to monitor the pneumatic canister that powers the projectiles. Williams differs from Rice et al in the use of a detachable device that fits onto the muzzle end of the barrel and additionally measures projectile velocity.
Since barrel temperature is known to be a critical factor in determining the rate of wear it is preferable that this parameter be monitored during firing if an accurate assessment of a weapon's condition is to be made. None of the patents cited have means to measure this temperature nor do they have a way to determine the number of rounds fired at a particular temperature. None address data storage and its presentation so that it can be easily interpreted by the user or by an armorer. Further shortcomings of the aforementioned devices is their inability to be easily adapted for use on different weapons. With the exception of Williams's device all are difficult to retrofit to a variety of small-arms. Furthermore, those devices that utilize inertial switches, thereby avoiding the miscounts that are inherent in other sensing systems, cannot easily be altered to accommodate accessories such as night-vision scopes or noise suppressors that substantially change the mass of a weapon.